Radio frequency identification (RFID) systems are employed in many applications, including tracking the movement of items in inventories. In such an application, an RFID tag or slave is attached to each item in the inventory. In one type of tracking system, an RFID reader or master queries the RFID tag on a predetermined time schedule to periodically track the inventory level. In some applications, the RFID reader reads the RFID tag of a tagged inventory item as the item passes a given location. For example, an RFID reader is located at an exit door of a store or a loading dock of a warehouse to track inventory that is leaving the store or warehouse.
The RFID tag and the RFID reader are directional antennae. The communication between the RFID reader and the RFID tag has the clearest signal with the highest signal to noise ratio when the directional antenna of the polarity of the emission from the RFID tag is aligned to the polarity of emission from the RFID reader. The efficiency of the communication between the RFID reader and the RFID tag degrades from maximum efficiency to minimum efficiency as a function of cos θ as θ goes from 0° to 90°. The angle θ is the angle between the polarity of the RFID reader and the polarity of the RFID tag.
The RFID tags can be applied in a haphazard manner to the inventory so that the RFID tag is not necessarily aligned to the RFID reader in the RFID system when the inventory is stacked for storage and/or transport to or from a given location. In this case, the communication between the RFID tag and the RFID master is degraded. In cases where the antenna of the RFID tag is perpendicular to the antenna of the RFID reader, the RFID reader may not receive transmissions from the RFID tag.
In order to prevent errors in inventory tracking, some RFID systems implement an RFID reader that changes its antenna orientation. An RFID reader that changes antenna direction has a higher complexity and higher cost than an RFID reader that has a set antenna orientation.
Therefore, there is a need in the art for improved communication between RFID tags and RFID readers.